The invention relates to a platey, fines deficient delaminated kaolin composition for particular use as a lightweight coating (LWC) for groundwood paper and comprising platelets produced by grinding a coarse fraction or a whole fraction of the kaolin crudes to a shape factor ranging from about 13 to about 18 where there is a substantial amount of platelets and an insubstantial amount of ultrafines.
It is well known in the paper industry that a wide variety of pigments, such as titanium dioxide, calcium carbonate, talc, synthetic silicates, and clays such as bentonite and kaolin, are suitable for use as paper fillers and/or coatings. Kaolin, a natural occurring hydrated aluminate silicate, is presently the most widely utilized and is available in a range of particle sizes and brightness, as well as being either delaminated or non-delaminated.
Kaolin clay pigments are obtained from kaolin crudes. The crudes contain kaolin particles, oversize (grit) and fine and coarse particle size impurities. Some of the impurities (e.g., fine ferruginous or titaniferous impurities) impart undesirable color to the clay. Other impurities have an undesirable effect on the rheology of the kaolin.
The kaolin portion of kaolin crudes is xe2x80x9cpolydispersexe2x80x9d in the sense that the particles occur over a range of sizes and shapes. Thus, a kaolin crude will not contain particles of a single size such as, for example, particles all of which are 2 micrometers. Typically a degritted kaolin crude will contain particles ranging in size from submicron or colloidal to particles 20 micrometers or larger.
Kaolins from different deposits, or even from different zones in the same deposit, can vary widely in the content of impurities, particle size distribution as well as the morphology of the kaolin particles. In general, crude kaolin particles are composed of individual platelets, and stacks or booklets concentrated in the greater than 2 micron faction. Particle sizes of kaolins are conventionally determined by sedimentation using Stokes law to convert settling rates to particle size distribution, and assume a spherical particle shape for the kaolin particles. Hence, the use of the conventional term xe2x80x9cequivalent spherical diameters (e.s.d.)xe2x80x9d to designate particle size.
Hydrous kaolin is white in color, has a fine particle size, is relatively chemically inert, and makes an ideal low cost paper filler. Calcined (anhydrous) kaolin is also available for use as a paper filler and can impart greater opacity to paper than the hydrous kaolin, but has the serious disadvantage of being more abrasive.
Prior art kaolin paper fillers and coatings are typically produced by a beneficiation process which typically consists of fractionating in a continuous centrifuge to remove oversize material followed by leaching to remove iron-based colored compounds. In the leaching process the kaolin is acidified with HSSO4 to a pH of 3.0 to solubilize the iron. Sodium hydrosulfite is then added to reduce the iron to a more soluble ferrous form which is removed during the dewatering process. The flocculated clay, generally at approximately 30% solids by weight, is then filtered, such as by dewatering on a rotary vacuum filter to a solids level approximately 60% by weight. The filter cake is then dried or redispersed with additional dry clay if it is to be sold as approximately 70% by weight solids slurry. To produce high brightness products, i.e., fillers having a brightness index greater than 90, impurities may be removed from the kaolin clay through flotation or magnetic separation. To produce a delaminated product, the coarse fraction from the initial centrifugation is ground in sand, grinders to shear the stacks of platelets normally found in kaolin and thereby produce individual particles having an equivalent spherical diameter less than 2 microns.
It is well appreciated in the art that kaolin clay pigments must have certain Theological and optical properties to be suitable for use in paper manufacture as paper coatings or paper fillers. The kaolin clay pigment must be available as a high solids suspension typically having a clay solids content of about 50% to about 70% by weight, but still exhibiting a viscosity low enough to permit efficient and economical pumping, mixability with other filler or coating components, and application to the paper. Additionally, it is of utmost importance that the kaolin pigment exhibit certain optical properties, namely high brightness, high gloss, and high opacity.
The influence of particle size distribution upon the optical properties of kaolin pigments, has long been appreciated in the art. For example, in U.S. Pat. No. 2,992,936, Rowland discloses that a kaolin clay product having the following particle size distribution (in terms of equivalent spherical diameter, e.s.d.) will consistently show improved brightness, gloss and opacity when used as a paper coating clay:
99-100% by wt. less than 5 microns e.s.d.
98-100% by wt. less than 4 microns e.s.d.
88-100% by wt. less than 1.7 microns e.s.d.
85-97% by wt. less than 1.5 microns e.s.d.
70-84% by wt. less than 1.0 microns e.s.d.
25-37% by wt. less than 0.5 microns e.s.d.
10-15% by wt. less than 0.3 microns e.s.d.
Rowland further discloses producing such a controlled particle size kaolin product by first degritting a kaolin clay slurry, thence passing the degritted kaolin clay slurry at 21% solids by weight through a Sharples centrifuge at 400 cc per minute at 6300 r.p.m. and then recentrifuging the overflow effluent at the same rate and r.p.m. The final overflow effluent represented a cut taken off the fine end of the degritted clay slurry and amounted to 22% by weight of the degritted clay slurry. The degritted clay slurry remaining after removal of this 22% fine cut, i.e., the combined underflows from the two centrifugation steps, was reslurried to about 20% solids with 0.15% sodium hexametaphosphate and allowed to settle. The sedimented coarse clay which amounted to about 48% of the degritted clay slurry, was discarded leaving about 30% by weight of the original degritted kaolin clay slurry as an intermediate product to be subjected to further treatment via bleaching, filtering and drying to yield a commercial coating clay product.
In a paper entitled xe2x80x9cChemically Induced Kaolin Floc Structures for Improved Paper Coatingxe2x80x9d, presented at the 1983 TAPPI Coating Conference, W. H. Bundy et al. disclosed an improved high bulking paper coating pigment, referred to as 1089, which comprises a chemically modified kaolin produced by the Georgia Kaolin Company, Inc. and marketed under the trade name Astra-Lite. Structures of optimum functionality are said to be derived by chemically treating a base kaolin clay having a particle size distribution wherein from about 80% to 93% by weight of the kaolin particles are less than 2 microns e.s.d. to selectively flocculate a portion of the submicron fines therein thereby aggregating a portion of these fines on the surface of larger kaolin platelets and effectively inactivating large portion of colloidal particles. Such a chemically modified kaolin coating pigment derived from a base kaolin wherein 92% by weight particles under 2 microns is presented by Bundy et al. having a particle size distribution as follows:
99% by wt. less than 5 microns e.s.d.
97% by wt. less than 3 microns e.s.d.
90.5% by wt. less than 2 microns e.s.d.
65.5% by wt. less than 1 microns e.s.d.
31.5% by wt. less than 0.5 microns e.s.d.
12.5% by wt. less than 0.3 microns e.s.d.
5.5% by wt. less than 0.17 microns e.s.d.
Such a chemically flocculated kaolin coating pigment may be produced, for example, as disclosed,in U.S. Pat. Nos. 4,075,030; 4,076,548; or 4,078,941, by selectively flocculating a base kaolin clay with the addition: of either a low molecular weight (less than 1,000,000) organic flocculant such as a polyfunctional amine, e.g., ethylene diamine or hexamethylene diamine, or long carbon chain amine, with or without citric acid and, optionally, in the presence of fine mica below 150 mesh in size.
There is disclosed in U.S. Pat. No. 4,738,726 an opacifying pigment composition suitable for use as a paper filler or coating which consists essentially of particles of hydrous kaolin clay flocculated with a controlled minor amount of a cationic polyelectrolytic flocculant, e.g., a quaternary ammonium polymer salt or a diallyl ammonium polymer salt. The base kaolin clay is selected to have a particle size distribution prior to flocculation wherein less than 35% by weight are finer the 0.3 microns, i.e., colloidal.
U.S. Pat. No. 4,943,324 to Bundy et al. discloses a high bulking uncalcined kaolin pigment suitable for use as a filler in paper and which exhibits improved opacifying efficiency. The composition essentially consists of kaolin particles which have not only been treated with an amine but also upon which aluminum hydroxide has been precipitated, and of which less than about 20% by weight have a particle size less than 0.3 microns in equivalent spherical diameter. A method is provided for producing the pigment from a hydrated kaolin clay having a brightness of about 89. The selected starting clay is dispersed in water and subjecting as a slurry to delamination, defining, surface treatment with an amine and aluminum sulfate, and leaching. The treated kaolin slurry is subsequently rinsed and filtered to produce a filter cake which is redisposed to yield a commercial paper filler.
U.S. Pat. No. 5,085,707 to Bundy et al. discloses a defined and delaminated kaolin composition exhibiting high opacity which is produced by defining and delaminating a base kaolin clay so as to reduce its colloidal content. The defined and delaminated kaolin composition may also be treated with hexamethylene and aluminum sulfate to further enhance opacity. Whether treated or untreated, the defined and delaminated composition functions superiorly in paper coating formulations, alone or blended with known coating clays, improving the opacity, print gloss, sheet gloss and printability of paper sheet coated therewith.
U.S. Pat. No. 5,168,083 discloses a high opacity defined kaolin clay product having a relatively narrow particle size distribution, low colloidal particle content and an average particle surface area of less than about 30 square meters per gram. This product is prepared by defining an aqueous kaolin clay slurry via controlled centrifugation to remove a substantial portion of the colloidal particles therein. Prior to being subjected to centrifugation, the aqueous kaolin clay slurry is prepared so as to improve the defining process achieved via centrifugation by first subjecting the aqueous kaolin clay suspension to scrub grinding so as to break up agglomerates into individual kaolin particles without substantial delamination of the kaolin clay particles. Thence, the mechanically dispersed kaolin clay suspension is dispersed to its optimum level by the addition of a chemical dispersant, most advantageously sodium hexametaphosphate, and dilution water is admixed into the aqueous kaolin clay suspension to reduce the solids content thereof to a level less than about 18% solids by weight, and preferably to about 50% to about 15% by weight, prior to centrifugation. This patent discloses further processing the underflow aqueous kaolin clay suspension after centrifugation in a manner which prepares the kaolin particles for use as a paper coating clay or for use as paper filling applications.
Delamination generally refers to the operation of subjecting the naturally occurring kaolin stacks in the aqueous clay slurry to shearing force thereby reducing the kaolin stacks or booklets to thin platelets. This shearing action may take place in a sand grinder or carried out in devices such as ball or pebble mills, extruders, or rotor-stator colloid mills. During mechanical delamination, large kaolin particles are disaggregated into smaller particles having a high aspect ratio, principally by parting clay crystals along basal cleavages resulting in a narrowing of the particle size distribution of the delamination particles.
The kaolin pigments produced by mechanical delamination are commercially made and marketed by various producers with processing facilities located in Georgia and South Carolina, using as raw material sources, the Cretaceous and Tertiary kaolin deposits of the region. It is believed that most of these commercially available delaminated kaolin pigments do not possess the unique desirable combination of high opacification and the low viscosity characteristics of some undelaminated pigments.
U.S. Pat. Nos. 5,169,443 and 5,411,587 to Willis et al. and assigned to Engelhard Corporation each disclose a paper coating kaolin pigment comprising mechanically delaminated kaolin particles wherein the pigments possess the opacification, smoothness, and printability advantages of conventional delaminated kaolin pigments but have desirably low viscosity and gloss not characteristic of conventional delaminated kaolin pigments. The kaolin pigment produced by the teachings of U.S. Pat. Nos. 5,169,443 and 5,411,587 involve generally the delamination of a feed material consisting of a mixture of coarse fraction of kaolin crudes or whole crude fractions with fines. The delamination level of these prior art pigments are achieved by controlling the particle size of the kaolin clay.
There is a need to provide a delaminated kaolin pigment obtained from the Cretaceous and Tertiary kaolin deposits of Georgia and South Carolina which has a low viscosity at high solids concentration (i.e. xcx9c70%), which feature is desirably characteristic of undelaminated pigments, and which is deficient in the amount of ultra-fine particles.
The invention meets this need. The invention provides a process for producing a low viscosity delaminated clay pigment from the kaolin deposits of Georgia and South Carolina whose delamination level can be considered as being controlled particularly by particle shape of the clay. The process involves grinding a kaolin crude having a particle size distribution where about 30 to 60% by weight is less than 2 microns and 20% or less is less than 0.25 micron and a shape factor less than 10 and grinding this clay to a shape factor ranging from about 10 to about 30, preferably from about 13 to about 18, and most preferably 15, and then classifying to obtain a particle size distribution where 80% by weight is less than 2 microns in equivalent spherical diameter (e.s.d.). This kaolin clay to be delaminated can be a whole crude fraction or a coarse fraction reject which may or may not be combined with a fine fraction and which coarse fraction may be obtained from a circuit used to produce any of the many commercial products available in the market place. Such coarse fraction reject may be that obtained from a circuit which produces what is generally classified as a high opacity, defined kaolin clay with a relatively narrow particle size distribution and marketed by the assignee of the present invention, ECC International Inc., as a paper coating or a paper filler disclosed in the above U.S. Pat. Nos. 4,943,324; 5,085,707; and 5,168,083.
These novel pigments have a G.E. brightness ranging from about 88 to about 92; are deficient in ultra-fine particles where less than 20% by weight are less than 0.25 micron e.s.d; and are fluid at a high solids concentration, i.e. 70%. The kaolin clay slurry of the invention at a solids level of about 70% by weight has a low shear Brookfield viscosity below 500 cps, and a high shear Hercules viscosity of about 18 dyne-cm above 200 ppm using an xe2x80x9cAxe2x80x9d bob. The product produced by the invention is generally comprised of platelets with very few stacks and a small percentage of fines which is especially useful for high-spec coating of lightweight groundwood paper. The pigments of the invention may be used as single pigments or in blends with other kaolin products or other minerals in coating formulations to coat paper and paperboard.
It is, therefore, an object of the invention to provide a lightweight coating (LWC) for a groundwood paper product which comprises kaolin clay particles having a shape factor ranging between 13 and 18.
It is a further object of the invention to provide a process for preparing a coating for a lightweight groundwood paper based on the particle shape of the kaolin clay particles.
A further object of the invention is to provide a method for controlling the degree of delamination of kaolin clay particles for use as a coating on lightweight goundwood paper products. More particularly, the invention involves measuring the shape factor of the kaolin particles and grinding the particles until the shape factor ranges between 13 and 18 and capable of producing a high solids concentration slurry of 70% by weight.
These and other objects and advantages of the present invention will be better appreciated and understood in reading the detailed description of the preferred embodiments.
The invention relates to the manufacturing of a delaminated kaolin clay product for particular use as a coating for light weight groundwood (LWC) paper. Preferably, during the processing of the kaolin clay, the crude is subjected to a grinding step where the shape factor is brought from less than 10 up to a shape factor ranging from about 10 to 30, preferably from about 13 to 18, and most preferably 15. The shape factor is equated to the morphology of the pigment, and is measured by a device disclosed in U.S. Pat. Nos. 5,128,606 and 5,576,617, the disclosures of which are incorporated herein by reference. These patents, are assigned to ECC International Limited, the parent company of the assignee of the present invention. xe2x80x9cShape factorxe2x80x9d is a measure of an average value of the ratio of mean particle diameter to particle thickness for a population of particles of varying size and shape as measured using the electrical conductivity method and apparatus described in the aforesaid U.S. Pat. No. 5,128,606 and using the equation derived in the patent specification of U.S. Pat. No. 5,128,606. xe2x80x9cMean particle diameterxe2x80x9d is defined as the diameter of a circle which has the same area as the largest face of the particle. Generally, if the shape factor is between 0 and 10, then the kaolin clay is composed substantially of thick stacks or booklets, where the e.s.d. is 2 to 5 xcexc. This may be considered as being too xe2x80x9cblockyxe2x80x9d for some applications. If the shape factor is between 10 and 30, the clay is composed substantially of individual thick platelets, which may be considered as being xe2x80x9cplatyxe2x80x9d. If the shape factor is between 30 and 60, the clay is composed of numerous thin platelets, which maybe considered as xe2x80x9ctoo platyxe2x80x9d for many practical commercial applications, such as coating for paper products.
The process of the invention involves grinding a kaolin crude with a particle size distribution where about 30 to 60% by weight is less than 2 microns and less than 20% by weight is less than 0.25 micron and with a shape factor less than 10 (the kaolin clay is xe2x80x9cblockyxe2x80x9d), and grinding the kaolin clay to a shape factor ranging from about 10 to 30, more preferably from about 13 to 18, and most preferably about 15, and then classifying the clay to obtain a particle size distribution where about 80% to about 90% by weight is less than 2 microns e.s.d. This crude used in the invention may be a whole crude, a coarse fraction of a whole crude, or a coarse fraction of a reject from a circuit used to produce any of the commercial clay products preferably having a shape factor between about 0 and 10 and a particle size distribution such that about 30% by weight to 60% by weight is less than 2 microns and  less than 20% by weight is  less than 0.25 micron e.s.d.
In the present application, all values for the particle size distributions (p.s.d.) are given in equivalent spherical diameters (e.s.d.).
A typical process for beneficiating a crude kaolin used in the invention may involve blunging the clay crude with water and dispersing it in a conventional manner in a commercially available blunging apparatus to produce an aqueous slurry of crude kaolin clay typically having a solids content ranging from about 20% to 70% by weight solids. This aqueous kaolin clay slurry may next be degritted, as in conventional practice, to remove substantially all particles in excess of 44 microns (i.e.,+325 mesh) equivalent spherical diameter (e.s.d.) The degritting of the crude kaolin slurry may be carried out by first passing the crude kaolin slurry through a screening device, such as a sandbox, to remove the coarsest material in the slurry and then fractionating the screened kaolin clay slurry on a classifier or Bird centrifuge to separate a coarse fraction comprising particles greater, than about 5 microns e.s.d. therefrom, which coarse fraction which generally may have been discarded, can be used as the feed material or part of the feed material in the invention.
The feed material for the invention can be made into an aqueous slurry typically having a solids content ranging from about 30% to about 60% by weight solids. This aqueous slurry or slip is then passed through the magnetic separator to remove a substantial portion of the iron impurities and produce a relatively low content, degritted clay slurry in the separator. In the separator, the slip may be retained for about 2 to 3 minutes at about 2,000 gause. From the separator, the slip is passed through an attrition grinder. Preferably, the particle size distribution of the kaolin particles going into the grinder range between 30 to 60% by weight  less than 2 microns and less than 20% by weight  less than 0.25 xcexcm and the shape factor is less than 10. In the grinder, the particles are delaminated and ground to a shape factor ranging from between 13 and 18. The slip from the grinder is then passed through a classifier to where about 80 to 90% by weight of the particles are less than 2 microns.
From the classifier, the slip is then bleached or leached to remove iron-based colored compounds. In the leaching process,the kaolin clay maybe acidified with H2SO4 to a pH of 3.0 to solubilize the iron. Sodium hydrosulfite is then added to reduce the iron to a more soluble ferrous form which is removed during the dewatering process. The slip from the leaching process then, in a conventional manner, is flocculated generally at approximately 30% solids by weight then filtered, rinsed and spray dried. The filter may be a rotary vacuum filter where the kaolin slurry is dewatered to a solids level of approximately 50 to 60% by weight and then dried to about 70% by weight solids.
Delamination as used in the invention refers to the operation of subjecting the naturally occurring kaolin stacks in the aqueous clay slurry to a shearing force thereby reducing the kaolin stacks to thin platelets. Delamination can be carried out by subjecting an aqueous slurry of stacked kaolin particles to shearing action in a sand grinder, ball or pebble mills, extruders, or rotor-stator colloid mills. The invention is described herein where the delamination occurred by use of an attrition grinder, however, it is to be appreciated that the various shearing devices in the art with the various media may be used.
The final kaolin pigment produced by the invention has a G.E. brightness between 88 and 92, and preferably 90.2, and a particle size distribution where 80% by weight of the pigments are less than 2 microns; and 10 to 20% by weight are less than 0.25 micron. The final product pigments of the invention are capable of forming clay-water slurry at 70% solids which have a low shear viscosity below 1000 cps, preferably below 500 cps when measured by the Brookfield Viscometer at 20 rpm. The high,shear Hercules viscosity for. this slurry is at 18 dyne-cm at above 200 rpm, using the xe2x80x9cAxe2x80x9d bob.
In the invention, it is believed that the delamination of kaolin stacks is essentially complete when the clay particles are ground to a shape factor of about 15.
Further grinding to a shape factor greater than 20 may cause attrition of individual platelets and may result in reducing the plate face dimensions or producing ragged edges on the individual platelets, both of which are undesirable for this kaolin product for particular applications as a coating on lightweight groundwood paper.